The present invention relates to a process for the liquefaction of natural gas and more particularly the liquefaction of natural gas to LNG (at atmospheric pressure) that does not require the use of external refrigerants.
Natural gas is an increasingly used fuel source throughout the world. Consequently, efforts for its production continue to grow in remote areas of the world where safe transportation of the natural gas to distant markets is impractical or requires significant capital expense. Where pipeline transportation of natural gas is not available or practical, liquefaction of natural gas is currently practiced as a cost effective option for transporting natural gas to worldwide markets.
As used throughout the specification, natural gas is understood to mean raw natural gas or treated natural gas. Raw natural gas primarily comprises light hydrocarbons such as methane, ethane, propane, butanes, pentanes, hexanes and impurities like benzene, but may also comprise small amounts of non-hydrocarbon impurities, such as nitrogen, hydrogen sulfide, carbon dioxide, and traces of helium, carbonyl sulfide, various mercaptans or water. Treated natural gas primarily comprises methane and ethane, but may also comprise a small percentage of heavier hydrocarbons, such as propane, butanes and pentanes.
As used throughout the specification, liquefied natural gas (xe2x80x9cLNGxe2x80x9d) is understood to mean natural gas that is reduced to a liquefied state at or near atmospheric pressure. As used herein, near atmospheric pressure is generally understood to mean no more than about 25 psia, commonly not more than about 20 psia, and often not more than about 15 psia.
The liquefaction of natural gas is generally accomplished by reducing the temperature of natural gas to a liquefaction temperature of about xe2x88x92240xc2x0 F. to about xe2x88x92260xc2x0 F. at or near atmospheric pressure. This liquefaction temperature range is typical for many natural gas streams because the boiling point of methane at atmospheric pressure is about xe2x88x92259xc2x0 F. In order to produce, store and transport LNG, conventional processes known in the art require substantial refrigeration to reduce and maintain natural gas at its liquefaction temperature. The most common of these refrigeration processes are: (1) the cascade process; (2) the single mixed refrigerant process; and (3) the propane pre-cooled mixed refrigerant process.
A cascade process produces LNG by employing several closed-loop cooling circuits, each utilizing a single pure refrigerant and collectively configured in order of progressively lower temperatures. The first cooling circuit commonly utilizes propane or propylene as the refrigerant, the second circuit may utilize ethane or ethylene, while the third circuit generally utilizes methane as the refrigerant.
A single mixed refrigerant process produces LNG by employing a single closed-loop cooling circuit utilizing a multicomponent refrigerant consisting of components such as nitrogen, methane, ethane, propane, butanes and pentanes. The mixed refrigerant undergoes the steps of condensation, expansion and recompression to reduce the temperature of natural gas by employing a unitary collection of heat exchangers known as a xe2x80x9ccold box.xe2x80x9d
A propane pre-cooled mixed refrigerant process produces LNG by employing an initial series of propane-cooled heat exchangers in addition to a single closed-loop cooling circuit, which utilizes a multi-component refrigerant consisting of components such as nitrogen, methane, ethane and propane. Natural gas initially passes through one or more propane-cooled heat exchangers, proceeds to a main exchanger cooled by the multi-component refrigerant, and is thereafter expanded to produce LNG.
Most LNG liquefaction plants utilize one of these natural gas liquefaction processes. Unfortunately, the construction and maintenance of such plants is expensive because of the cost of constructing, operating and maintaining one or more external, single or mixed refrigerant, closed-loop cooling circuits.
Another penalty associated with external closed-loop cooling circuits is that such circuits require the use and storage of highly explosive refrigerants that can present safety concerns. Refrigerants such as propane, ethylene and propylene are explosive, while propane and propylene, in particular, are heavier than air further complicating dispersion of these gases in the event of a leak or other equipment failure. This is of particular concern during the offshore production and transport of LNG by ocean going vessels or other floating vessels because of: (1) the large amount of refrigerants that must be stored in order to maintain the liquefaction temperature of natural gas; and (2) the close proximity of these refrigerants to the ships crew.
Consequently, there is a need for a cost efficient means for safely producing, storing and transporting LNG to commercial markets around the world. The current methods have only partially succeeded in providing a safe yet cost effective process.
One such effort is U.S. Pat. No. 5,755,114 issued to Foglietta, which discloses a hybrid liquefaction cycle for the production of LNG. The Foglietta process passes a pressurized natural gas feed stream into heat exchange contact with a closed-loop propane or propylene refrigeration cycle prior to directing the natural gas feed stream through a turboexpander cycle to provide auxiliary refrigeration. The Foglietta process can be implemented with only one closed-loop refrigeration cycle, as opposed to cascade type mixed refrigerant systems currently used to produce atmospheric LNG. However, the Foglietta process still requires at least one closed-loop refrigeration cycle comprising propane or propylene, both of which are explosive, not easily dispersed and must be stored on the vessels that transport the Foglietta product.
U.S. Pat. No. 3,360,944 to Knapp et al. produces LNG by separating a natural gas feed stream into a major stream and a minor stream, cooling the major and minor streams to produce a liquid component, and thereafter using a substantial portion a the liquid component as a refrigerant for the process. The liquid component is vaporized while undergoing heat exchange, compressed and discharged from the process. The Knapp process results in only a minor portion of the natural gas feed stream processed into LNG.
U.S. Pat. No. 6,023,942 to Thomas et al. discloses a process for producing a methane-rich liquid product having a temperature above about xe2x88x92112xc2x0 C. (xe2x88x92170xc2x0 F.) at a pressure that is sufficient for the liquid product to be at or below its bubble point. The resulting product is a pressurized liquid natural gas (xe2x80x9cPLNGxe2x80x9d), which has a pressure substantially above atmospheric pressure. While the Thomas et al. process can be implemented without external refrigeration, the product is pressurized requiring the use of specially designed heavy, thick-walled containers and transports (e.g., a PLNG ship, truck or railcar). This higher pressure, heavier walled equipment adds substantial weight and expense to any commercial project. The PLNG consumer will also require additional liquefaction, transport, and storage equipment to consume the PLNG, adding further cost to the supply and demand value chain.
U.S. Pat. No. 3,616,652 to Engal discloses a process for producing LNG in a single stage by compressing a natural gas feed stream, cooling the compressed natural gas feed stream to produce a liquefied stream, dramatically expanding the liquefied stream to an intermediate-pressure liquid, and then flashing and separating the intermediate-pressure liquid in a single separation step to produce LNG and a low-pressure flash gas. The low-pressure flash gas is recirculated, substantially compressed and reintroduced into the intermediate pressure liquid.
While the Engal process produces LNG without the use of external refrigerants, the process inefficiently utilizes its limited refrigeration capacity upon the entire process stream without conjunctive use of multiple separation steps to offset this severe cooling requirement. Furthermore, the Engal process inefficiently expands its process stream pressure to a level that results in a substantial and highly inefficient recompression of its flash gas. Consequently, the Engal process yields a small volume of LNG compared to the amount of work required for its production, thus reducing the cost viability of the process.
Although these processes provide great advances in the art, none have satisfied the need for a process that is safe yet cost effective for manufacturing LNG.
We have now found that processing a single stream of natural gas into LNG while deriving the refrigeration for the process from flash gases separated in multiple sequential separation steps results in enhanced LNG production and reduced equipment costs compared to processes that produce LNG by splitting the feed stream into major and minor natural gas streams and producing a liquid component for its refrigeration requirements.
We have also found that staging the degree of expansion of a high-pressure natural gas feed stream across multiple cooling/expansion/separation steps or cooling stages enhances the production of LNG while reducing the power consumption for its production compared to LNG processes that dramatically reduce the pressure of a high-pressure natural gas feed stream across a single expansion step or cooling stage.
We have also found that processing a single stream of natural gas into LNG by utilizing a plurality of cooling stages comprising two or more separation steps in conjunction with at least an equal number of expansion steps substantially reduces the refrigeration requirements for the process, thus enhancing the production of LNG while reducing the equipment costs compared to processes that produce LNG without the use of such linked multiple expansion and separation steps.
Therefore, the present invention is directed to a process for producing LNG by directing a feed stream comprising natural gas to a cooling stage that (a) cools the feed stream in at least one cooling step producing a cooled feed stream, (b) expands the cooled feed stream in at least one expansion step by reducing the pressure of the cooled feed stream producing a refrigerated vapor component and a liquid component, and (c) separates at least a portion of the refrigerated vapor component from the liquid component wherein at least a portion of the cooling for the process is derived from at least a portion of the refrigerated vapor component; and repeating steps (a) through (c) one or more times until at least a substantial portion of the feed stream in the first cooling stage is processed into LNG wherein the feed stream in step (a) comprises at least a portion of the liquid component produced from a previous cooling stage.
In another embodiment, the present invention is directed to a process for producing LNG by directing a feed stream comprising natural gas to a cooling stage that (a) cools the feed stream in at least one cooling step producing a cooled feed stream, (b) expands the cooled feed stream in at least one expansion step by reducing the pressure of the cooled feed stream producing a refrigerated vapor component and a liquid component, and (c) separates at least a portion of the refrigerated vapor component from the liquid component wherein at least a portion of the cooling for the process is derived from at least a portion of the refrigerated vapor component; and repeating steps (a) through (c) one or more times wherein the feed stream in step (a) comprises at least a portion of the liquid component produced from a previous cooling stage wherein the inlet pressure of the feed stream in step (b), as measured in psia, is at least xc2xd the inlet pressure of the feed stream in step (a), as measured in psia, of the immediately preceding cooling stage provided that the inlet pressure of said feed stream in step (a) is at least 150 psia.
The present invention provides for a cost effective process for producing LNG that does not require the costly capital necessary for closed-loop refrigeration circuits.
The present invention also provides a cost effective process for producing LNG that does not require high-pressure containers and transport equipment for handling highly pressurized LNG product nor does the process require customers to erect special handling facilities and equipment required for consuming high pressure LNG.
The present invention also provides a process for producing LNG that does not require explosive external refrigerants during the manufacture, storage or transportation of LNG.
The present invention also provides for a simple and compact design option for the production of LNG facilitating implementation of the process at locations where plot space is at a premium or unavailable.
The present invention also provides a process for producing fuel gas for internal process consumption, while maintaining a high rate of LNG production and efficient power consumption for the process.
The present invention also permits manufacture of a high quality LNG product having low concentrations of inert components, such as nitrogen, and the ability to remove NGL components, such as ethane, propane, butanes and pentanes and heavier components, and Benzene from the feed.